cmd/rundockerbuildlet: retrieve host type from metadata on EC2
cmd/rundockerbuildlet: retrieve host type from metadata on EC2

When rundockerbuildlet is running on an EC2 instance, the
configuration should be pulled in from the metadata. This is part of
the effort to move some configuration for builders over to the
coordinator.

For golang/go#36841

Change-Id: Icbf0e73301faadd06c87f02aa1172bd0e6f87624
Reviewed-on: https://go-review.googlesource.com/c/build/+/252317
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build